Optical detection is the determination of the presence and/or concentration of one or more target species within a sample by illuminating the sample with optical radiation and measuring optical absorption, induced fluorescence, and/or optical scattering by the sample. Optical detection has a wide variety of applications, including spectroscopy and liquid chromatography, and a correspondingly wide variety of optical detection methods are known. Cavity enhanced optical detection entails the use of a passive optical resonator, also referred to as a cavity, to improve the performance of an optical detector. Cavity enhanced absorption spectroscopy (CEAS) and cavity ring down spectroscopy (CRDS) are two of the most widely used cavity enhanced optical detection techniques.
The intensity of single-mode radiation trapped within a passive optical resonator decays exponentially over time, with a time constant τ, which is often referred to as the ring-down time. In practice, it is desirable to ensure that only a single resonator mode has an appreciable amplitude, since excitation of multiple resonator modes leads to multi-exponential radiation intensity decay (i.e., multiple time constants), which significantly complicates the interpretation of measurement results. The ring-down time T depends on the cavity round trip length and on the total round-trip optical loss within the cavity, including loss due to absorption and/or scattering by one or more target species within a sample positioned inside the cavity. Thus, measurement of the ring-down time of an optical resonator containing a target species provides spectroscopic information on the target species. Both CRDS and CEAS are based on such a measurement of τ.
In CRDS, an optical source is usually coupled to the resonator in a mode-matched manner, so that the radiation trapped within the resonator is substantially in a single spatial mode. The coupling between the source and the resonator is then interrupted (e.g., by blocking the source radiation, or by altering the spectral overlap between the source radiation and the excited resonator mode). A detector typically is positioned to receive a portion of the radiation leaking from the resonator, which decays in time exponentially with a time constant τ. The time-dependent signal from this detector is processed to determine τ (e.g., by sampling the detector signal and applying a suitable curve-fitting method to a decaying portion of the sampled signal). Note that CRDS entails an absolute measurement of τ. Both pulsed and continuous wave laser radiation can be used in CRDS with a variety of factors influencing the choice. The articles in the book “Cavity-Ringdown Spectroscopy” by K. W. Busch and M. A. Busch, ACS Symposium Series No. 720, 1999 ISBN 0-8412-3600-3, including the therein cited references, cover most currently reported aspects of CRDS technology.
Single spatial mode excitation of the resonator is also usually employed in CEAS, (sometimes called integrated cavity output spectroscopy (ICOS)), but CEAS differs from CRDS in that the wavelength of the source is swept (i.e., varied over time), so that the source wavelength coincides briefly with the resonant wavelengths of a succession of resonator modes. A detector is positioned to receive radiation leaking from the resonator, and the signal from the detector is integrated for a time comparable to the time it takes the source wavelength to scan across a sample resonator mode of interest. The resulting detector signal is proportional to τ, so the variation of this signal with source wavelength provides spectral information on the sample. Note that CEAS entails a relative measurement of τ. The published Ph.D. dissertation “Cavity Enhanced Absorption Spectroscopy”, R. Peeters, Katholieke Universiteit Nijmegen, The Netherlands, 2001, ISBN 90-9014628-8, provides further information on both CEAS and CRDS technology and applications. CEAS is discussed in a resent article entitled Incoherent broad-band cavity-enchanced absorption spectroscopy by S. Fiedler, A. Hese and A. Ruth Chemical Physics Letter 371 (2003) 284–294.
In cavity enhanced optical detection, the measured ring-down time depends on the total round trip loss within the optical resonator. Absorption and/or scattering by target species within the cavity normally accounts for the major portion of the total round trip loss, while parasitic loss (e.g., mirror losses and reflections from intracavity interfaces) accounts for the remainder of the total round trip loss. The sensitivity of cavity enhanced optical detection improves as the parasitic loss is decreased, since the total round trip loss depends more sensitively on the target species concentration as the parasitic loss is decreased. Accordingly, both the use of mirrors with very low loss (i.e., a reflectivity greater than 99.99 per cent), and the minimization of intracavity interface reflections are important for cavity enhanced optical detection.
Cavity enhanced optical detection can be used for solid, liquid, aerosol, or gaseous samples. For gaseous samples, intracavity interfaces are typically not present, so there are no corresponding interface reflection losses to contribute to round trip parasitic loss. However, intracavity interfaces are typically present for solid or liquid samples. For example, contamination of the mirror surfaces by aerosols i.e., liquid and/or solid particulate containing gas samples can create problems so that these samples are generally enclosed in an intracavity cell. This cell will create interfaces (e.g., windows) within the optical resonator. Similarly, the boundaries of a solid sample are per se intracavity interfaces. Likewise, for a liquid sample contained in a flow cell present within a cavity, the interfaces between the liquid and the inner wall of the flow cell as well as the exterior wall surfaces of the flow cell are all intracavity interfaces. U.S. Pat. No. 6,452,680 teaches the minimization of intracavity reflection loss when examining solid or liquid samples by positioning the sample such that optical radiation circulating within the optical resonator is, insofar as possible, incident on the sample-induced interfaces at an angle approximating Brewster's angle and is p-polarized relative to these interfaces. Since reflection is minimized for p-polarized incidence on an interface at
Brewster's angle, this arrangement significantly reduces reflection-induced parasitic loss. FIG. 1a is a schematic illustration of this cell configuration.
A variation of the design shown in FIG. 1a is shown in FIG. 1b, which design is also known to the prior art (K. Snyder and R. N. Zare, “Cavity Ring-down Spectroscopy as a Detector for Liquid Chromatography” Analytical Chemistry, Vol. 75, p 3086–3091 (2003). In this design the liquid flow channel is tilted within the cell so that the light beam strikes each surface at the correct Brewster's angle for the specific interface (e.g. air→fused silica→liquid→fused silica→air). With the appropriate polarization of light, the interface reflections are minimized, thereby allowing the light to pass back and forth through the cell multiple times, resulting in a relatively long ring-down constant. In the example shown in FIG. 1b, using a fused silica cell and water as the sample liquid, angle e is 7.9° and angle a is 55.6° so that the light refracts through the cell, hitting each interface surface at approximately Brewster's angle for minimum reflection. The system shown in FIG. 1b provides some advantage relative to the arrangement of FIG. 1a in that by tilting the flow channel within the cell the light path is incident on all interfaces at approximately Brewster's angle.
However, the arrangements shown in FIGS. 1a and 1b both suffer from a drawback in that a change in the refractive index of the sample can cause the cavity to become misaligned and potentially unstable. A detailed discussion of cavity stability can be found in Chapters 19 and 20 of “Lasers” by A. E. Sigman, University Science Books Sausalito, Calif., 1986. Variability of the sample refractive index is especially pertinent for liquid chromatography applications, since the sample refractive index will frequently change as a separation proceeds, especially in the many cases where it is desirable to use various solvents having different refractive indices to perform sequential or different separations in the same instrument. In addition, variation of sample temperature and/or pressure can also cause changes in the sample refractive index. This same analysis applies to the situation where the light source wavelength λs is changed from the original design wavelength to λd (λd≠λs) In this case the light will travel along different paths in both the flow cell (e.g.,glass) and the fluid sample. This is a significant limitation if it is desired to use the system with a broadband or tunable source of radiation.